The measurement of a ductile surface or membrane is traditionally done by external application of a force to develop an understanding of the reaction to the surface or membrane to the application of a force. For the case of an elastic surface or membrane behavior, an example characterization may be drawn to a spring constant over a short range of motion, or in the case of a displacement in one direction followed by a non-linear displacement in the opposite direction, for a lossy system, a hysteresis effect may also be observed. There are several conditions of interest associated with the relationship between the membrane and an adjacent fluid or gel. In one condition, the membrane ductility dominates the measurement, and the effect of materials adjacent to the membrane such as air, fluid, or gel have no effect on the measurement, as the membrane characteristic dominates the measurement. In another condition, the membrane provides an interface and challenge surface for a surface stimulation, and the characteristics of the fluid or gel adjacent to the membrane are measured using the membrane, which otherwise does not change the characteristics being measured, as the membrane is closely coupled to the surface and underlying fluid such that displacements to the underlying fluid may be made through the surface or membrane without changing the characteristics to be measured. Such characterizations are widely performed using mechanical (stirring paddles) or other means in the food industry to measure the stiffness or other mechanical properties of various liquid or semi-solid food items, such as those with curing or culture times. For example, in the prior art of food science, it is desired to know when yogurt has reached a particular culture level by its mechanical properties, whereas the bacterial growth interval may be determined from prior batches and a fixed interval used. Similarly, the ripeness of a fruit or vegetable may be determined by its surface softness, elasticity, or other mechanical property, which may be performed by the application of a small force sufficient to produce measurable deflection or which causes surface or shear waves across the surface of the fruit or vegetable and measuring the minute surface or sub-surface deflections in response to that surface force. Accordingly, it is desired to measure the mechanical properties of food items using non-contact methods of measurement applied to the surface of the item to be characterized.
In the medical field, it may be desirable to measure the elasticity of a skin surface, a movable membrane, or organ surface. In one example, the eye globe may be measured by using a contact or optical method, to determine the deflection upon application of an air puff. In the prior art, the optical reflectivity characteristics of the eye may be measured as a deflection versus applied force. However, the optical interface of the eye and the various underlying structures may have resolution of motion limitations which are the consequence of the optical measurement system characterizing an optically transparent structure, where the intended target (such as the cornea) and surrounding structures (such as the ocular lens and vitreous fluid) have similar optical properties and are therefore difficult to resolve from each other. It is desired to use remote sensing and remote interrogation, such as through air, to measure the elasticity of the surface of an eye or other organ.
Ultrasound measurements are typically performed using a liquid medium as a coupling fluid, such that the ultrasound energy propagates from the transducer through the fluid as traveling acoustic waves, creating reflected ultrasound energy arising from target interfaces which have differences in acoustic index of refraction according to the transmission impedance mismatch between the coupling fluid and the target. The use of ultrasound has not been possible for non-contact displacement measurements resolving distances on the order of millimeter (mm) and sub-mm distance ranges because of the traditional requirement for coupling fluids between the transducer and target.
A method for the non-contact surface characterization of a surface through remote sensing of displacement in combination with the application of a surface stimulus is desired. It is further desired to use air as the medium for stimulation and measurement, without direct contact to the surface or membrane by either the stimulus or measurement device.